This formula should be an expression rather than a code as you have written.
The expression should evaluate to true or false as we have in SSRS reports as below:-
=IIf(taxper31.text == 0, true, false)
In your case you may try as below:-
You need to define a suppress formula for the object to accomplish this?
Right click on the text box >> Go to formula field >> Common tab >> suppress X2 formula
and type a formula like this:-
if {table.Field} = 0 then true else false
Hope this is of help to you.